Natsuko, the Sato family's best friend from their school days, comes to stay the night. Satoru, the son, is drawn in by Natsuko's charm, which is full of adult appeal. When Natsuko finds out that Satoru is still a virgin, she gently teaches him how to be a virgin, saying, "I'll be your first woman." Satoru is nervous and confused, but he successfully loses his virginity and, like a newly learned monkey, he desires Natsuko over and over again...